This invention concerns a prosthesis for resurfacing the glenoid cavity of the shoulder and to a method of attaching the same which provides improved fixation of the prosthesis to the bone.
The glenoid cavity is located on the upper external border of the scapula between the acromion process and the coracoid process, on a bony formation known as the scapula head. The glenoid cavity is a shallow, pear-shaped, articular surface whose longest diameter is from above downward, and its direction outward and forward. It is broader below than above, and at its apex is a slight impression, the supra-glenoid tubercle, to which is attached the long tendon of the biceps muscle. The cavity is covered with cartilage and its margins, slightly raised, give attachment to a fibro-cartilaginous structure, the glenoid ligament, by which its cavity is deepened.
The glenoid cavity articulates with a large, rounded head at the proximal end of the humerus, or upper armbone. The head is nearly hemispherical in form and is directed upward, inward, and a little backward. Its surface is smooth and coated with cartilage.
There are at least four types of forces applied to the glenoid cavity which should be accounted for in designing a glenoid prosthesis.
First, there is straight compression loading which occurs, for example, when a person is standing with one side facing the wall and his arm straight out and he is leaning on the wall with his arm. This generates a compressive load which is transmitted straight into the humeral glenoid joint. The glenoid joint is designed to readily accomodate this straight compression loading.
Second, is a rotational shear loading which would occur, for example, when a person makes circles with his arms next to his body. This shear loading is generally of lesser magnitude.
Third, superior loading of the glenoid occurs, for example, when a person sits in his chair and puts his hands on the arm rests and pushes down to assist in getting up out of the chair. A superior loading force is applied to the superior end of the glenoid cavity which creates a moment load on the inferior end of the glenoid cavity.
Fourth, an anterior-to-posterior loading occurs, for example, when a person is sitting at a desk and pushes away from the desk with his arms.
The known glenoid prosthesis designs fail to accomodate one or more of these forces to the same extent as the glenoid prosthesis of this invention.
For example, the Neer II Total Shoulder System has a glenoid resurfacing component made of ultra-high molecular weight polyethylene (UHMWPE) having a concave lateral articulating surface and a convex medial surface with longitudinal grooves and a roughly triangularized fin projecting from the medial surface. The component is applied to the bone by using a right-angle burr to machine a trough into the center of the glenoid cavity about one inch long and about one 1/8 inch wide to accomodate the fin. The burr may be angled outwardly to widen the bottom of the fin cavity to prevent pullout of the fin once cement has solidified around the fin in the cavity. The fin further includes a central aperture and thickness variations, apparently to enhance the cement fixation. A metal-backed supporting shell is provided in an alternative embodiment having transverse grooves along the medial surface.
The Neer II glenoid component has been shown to loosen over time (i.e., detach from the bone) due to a superior-inferior rocking motion of the component. Superior loading applied to the superior margin of the glenoid component creates a moment load at the inferior end of the glenoid component causing the convex medial surface to rock along the superior-inferior longitudinal axis. The fin fails to prevent such rocking motion which causes either the cement to break down and the component to pull out, or causes the bone to break and the component to pull out.
Another known device, the DePuy shoulder prosthesis, has a glenoid component consisting of a plastic insert attached to a metal base. The medial surface of the base is cone shaped and four screw holes are provided through the base. The base is fixed to the bone by pushing the cone shaped base into a mating cavity resected from the glenoid cavity and by screwing four screws through holes in the base and into the bone. Again, in many cases, this design fails to resist the superior-inferior rocking motion which leads to a loosening of the component from the bone.
Another known device is the Richards-Cofield shoulder prosthesis, also having a glenoid component with a plastic insert and metal base. Again, the metal base has a curved medial surface and thus requires the use of a reamer to prepare a suitably curved mating surface in the glenoid cavity. The metal base plate further includes a central projection which is press-fit into the bone and a pair of superior-inferior screw holes with medially extending bosses which receive bone screws. The polyethylene insert is attached to the metal base by means of a slot. There have been problems with the insert dislocating from the metal base.
It is an objective of this invention to provide a glenoid prosthesis which can be fixed more securely to the bone so as to prevent loosening of the prosthesis or bone fracture under applied loads.
Another objective is to provide a glenoid prosthesis having an increased area for bone fixation.
A further objective is to provide an all plastic glenoid prosthesis which can be securely fixed to the bone with cement.
A further objective is to provide a metalbacked glenoid prosthesis which can be used to attach bone grafts.
A still further objective is to provide a glenoid prosthesis requiring a minimum number of steps to manufacture.
Another objective is to provide a new method for attaching a glenoid prosthesis resulting in improved fixation which resists loosening.
Yet another objective of the invention is to provide a method for attaching a glenoid component which can be performed more quickly and easily than the known methods.